I know for sure one SIM works in another phone, but im quite sure the 2nd one does too..

All I did today was what I wrote in 1st post and updated 2 apps:
maecalc
qtlockscreen

honestly I think qtlockscreen is the culprit. as I was messing with its config file (just an *.ini) and then out of nowhere my phone starts asking for the lock code. I entered it press ok but it kept asking for it????????

Well you did say before that you have reflashed your N900 and you still don't get cellular connectivity. Unless you have qtlockscreen installed previously (and that was prior to maybe using something like osso-backup or backupmenu) It would have reinstalled/restored that package.

I am still going by the assumption that you had a clean N900 (without any restoration from osso-backup/backupmenu) and you do not have cellular connectivity.

after reflashing today (twice) I have not restored anything

FIXED!!!!!!!!!!!!!! I dont know exactly what worked but I left the battery and SIM out for at least 20 mins then tried a different battery and SIM I hadnt tried yet...

seems it was a problem with my SIM.. I put my partners SIM and battery in, when I booted it asked "go out of offline mode" (hadnt seen that yet today!) then it worked.. then, just out of curiosity, I put both my battery and SIM back in - not working again. THEN, one more test - my partners battery and my SIM - still not working. looks like I needa new SIM. thank f**k for that.

case closed.(?)

Im quite sure leaving the battery out for 20-30 mins fixed it.

its good to know that it got solved... i would like to suggest you to rename the topic as.... "SOLVED... SIM DETECTION FAIL" or something that suits better...
in that way it would be helpful for Others who get Similar kind Problem...